With a broad grin of delight, 12-year-old Roisin Griffiths of Londonderry in Northern Ireland is discussing what she does and doesn't like about life in these United States. She is learning what she knows from a short summer stay in Chicago.

She and 150 other 11- to 14-year-olds from troubled pockets of Northern Ireland are in the midst of a sunny picnic on the grassy grounds of Lincoln Park Zoo. They have just spent the morning here, patting the soft backs of everything from possums to screech owls.

Roisin's list of minuses is short. Only mosquitoes and strong thunderstorms are added to it. She bites into a bright red apple and begins to recite an endless list of pluses: hamburgers (''We don't have any McDonald's''), French fries, ice cream, movies like ''E. T.'' and ''Annie,'' the view of Chicago from the top of the Sears Tower.

Last but not least for Roisin, who comes from a Roman Catholic neighborhood, there is the newfound friend sitting beside her: 11-year-old Angela Dougherty, a Protestant from the same city of Londonderry. At home they probably never would have met. Here they have become the best of friends.

''At first all of these children were very conscious of who was Catholic and who was Protestant, but we've seen a tremendous breaking down of barriers,'' observes Robert O'Connor, the man largely responsible for the Irish visitors sitting here.

Mr. O'Connor, a chemical firm marketing director, came to this country from Dublin 18 years ago. Troubled by the Northern Irish problem and convinced that no immediate political solution was likely, O'Connor and his wife, Diane, decided they wanted to do something to try to change the fundamental attitudes underlying the conflict.

Six months ago they launched a nonprofit nondenominational group called the Irish Children's Fund Inc. Its express purpose: to bring children from strife-torn areas of Northern Ireland to this country for an educational and recreational change of scene.

The O'Connors began to talk about the idea with friends and to various small groups. Soon they had more host family volunteers ofevery religious and ethnic background than they could use. The families, by every means from bake sales to garage sales, set about raising the necessary $75,000 to finance the air fares and insurance for the children's seven-week trip.

Meanwhile, on the other side of the Atlantic, Maj. Robert W. Menary of Belfast, commander of the Northern Ireland division of the Salvation Army, contacted headmasters of Catholic and Protestant schools in that city and Londonderry. They were asked to select youngsters from tense, polarized neighborhoods who most deserved such a holiday.

Major Menary, who with five chaperones accompanied the children on the trip, says that one of the major accomplishments of theprogram took place shortly after the children were selected. Parents in each city were invited to an orientation session.

''In the normal course of events these parents would never mix, but in this case they all said the Lord's Prayer together,'' recalls Major Menary. ''If we'd been trying to arrange a trip for the children five miles down the road, it probably would have been a lot more difficult. But the wonderful privilege of coming to this country overcame a lot of the difficulties. . . . It essentially bridged the gulf of prejudice and hatred.''

Several of the visiting children, according to Major Menary, have ''strong terrorist backgrounds.'' Many have spent time throwing rocks at soldiers and police. Some have older brothers or fathers in jail and, at least on home turf, are proud of that fact. But some of the children also talk about the almost constant tension and fear they face in cities where bombings and gunfire are fairly routine.

''If a bomb went off over there,'' says Major Menary, pointing to a spot in the zoo about 50 feet away, ''these children probably wouldn't even bother to go see what had happened. They've grown up with terrorism. They think it's normal.''

One of the great hopes behind the program, of course, is that these youngsters will realize that a more peaceful way of life can also be normal.

''What surprises me here is the freedom,'' says 14-year-old Donna Dougall, a Protestant from Belfast. ''Everybody mixes. You can't mix at home because you'd be called a 'Catholic lover' and your friends would turn against you. And here there aren't any soldiers around and you're not searched everytime you go into a big store, as you are at home.''

The Northern Irish parents were told, but not guaranteed, that an effort would be made to place their youngsters with a family of the same faith. In fact , some mixing has gone on. Marie Lee, a mother of three children under five from Frankfort, Ill., says she and her husband John, who are Catholic, purposely asked to host Protestant youngsters. They drew two girls.

''My husband in particular felt this was a unique opportunity to demonstrate that a Catholic family can be loving, too,'' she explains.

While many children have gone on vacation trips with their host families, they have made several excursions as a group, including a trip to Springfield, Ill., to see Abraham Lincoln's home, a boat ride along Lake Michigan, and a Chicago White Sox game.

Irish chaperone Noel Moore, a physics teacher, says he thinks that the biggest impression made on all the children is the warm and friendly way in which American families have opened up their homes to them.

''I think the changes taking place are subconscious ones that may not be apparent for a few years,'' he says.

''This is really a drop in the ocean,'' agrees Major Menary. ''But we hope at least one boy or girl may change their way of life because of this experience and that somewhere lives may be saved. . . . It's an injection of goodness into their experience.''

Meanwhile, O'Connor, who has a long list of parents waiting to serve as host families, plans to continue the program next summer. He is also spearheading an effort to organize reunions of youngsters in Belfast and Londonderry youth centers.